Three stage ESD protection device

ABSTRACT

An MOS electrostatic discharge, ESD, protection circuit for protecting semiconductors from ESD damage is formed on a doped silicon substrate. The circuit includes three stages. The first stage includes a first MOSFET transistor and a grounded region formed in the substrate. The first MOS transistor has a source/drain circuit connected between the first node and ground, and has a control gate electrode connected to ground. The second stage includes a string of MOSFET transistors connected in a series string. The transistors in the string are in sufficient number to provide a circuit which will conduct at a high current level to protect the output circuit from overvoltage when the voltage exceeds a critical value. The third stage includes a third stage MOSFET device with a control gate connected to the second stage output and to the output of the circuit. The source and drain circuit of the third stage device are connected between the third node and the ground connection.

This is a division of application Ser. No. 08/507,143 filed Jul. 26,1995, now U.S. Pat. No. 5,593,911.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor devices and more particularly toprotection of semiconductor devices from problems attributable to excessenergy.

2. Description of Related Art

Static charge can build up in integrated circuits, such as MOSFETsemiconductor devices, to unacceptable levels in normal use. When thecharge reaches a critical level the static charge is discharged in theintegrated circuit, in an phenomenon known as ElectroStatic Discharge(ESD). The voltages reached during an ESD discharge event range as highas 1K Volts. Such voltages can damage the semiconductor circuitsincluding the semiconductor MOSFET devices, or the metal contactsassociated with the devices due to overheating caused by the excess heatthe device dissipates during and ESD event. The amount of power which isconverted into overheating is proportional to the square of the voltagereached; and the voltage reached is dependent on the breakdown voltageof the affected device.

Thick field oxide (FOX) devices have been used in input protectionschemes also, but the disadvantage they have is high turn on voltage,usually larger than 12 Volts

U.S. Pat. No. 4,821,096 of Maloney for "Excess Energy Protection Device"shows ESD protection devices for MOSFET devices, using PMOS devices asprotection devices, but there is no suggestion of using a series of MOSdevices to modify the protection device.

The Maloney patent describes the background of ESD, pointing out that itis known that by coupling to the protected device, devices which havelower breakdown voltages, the power dissipated will be decreased,reducing the effect of heat in device failure. By dissipating the excessenergy through a protecting device, the active device is not exposed tovoltage levels which can cause failure. Certain types of excess energyevents can adversely affect integrated circuits. Integrated circuits canbe affected by other energy events from other sources, for example,voltage and current surges. These events create excess electrical energyto be dissipated within the circuits. The energy discharge path is froman input/output pad to ground and can involve the initial stages of theintegrated circuit. If the excess energy event exceeds the oxidebreakdown voltage of a device in the initial stage, oxide breakdown,damage and even failure result. Additionally, metal/silicon contacts andPN junctions may be thermally damaged as a result of the heat dissipatedfrom the excess energy event.

To limit the impact of excess energy events, protection devices areinserted between the pad and the initial stage of the integratedcircuit.

One protection device consists of a pair of diodes coupled to the input.One diode is coupled to a high power supply voltage level and the otheris coupled to a low power supply voltage level or ground. A disadvantageof such a protection device is the excessively high voltage referencecoupled to the first transistor.

A second protection device consists of a grounded gate n-channeltransistor coupled through a resistor to a pad. This device is notalways effective in CMOS applications. Studies have shown that theresistor fails or that n-channel devices fail during high energy eventsbecause of alloy spiking of the N+ junction. In N-MOS processing, thisdefect can be overcome by utilizing deep N+ junctions.

However, in CMOS processing shallow N+ junctions are often employed. Asa result, the alloy spiking causes failure of N-channel protectiondevices. This defect can be overcome by enlarging the contact to gatespacing, but this requires additional silicon area which adds to thesize of the integrated circuit.

U.S. Pat. No. 4,527,213 of Ariizumi for "Semiconductor IntegratedCircuit Device with Circuits for Protecting an Input Section Against anExternal Surge" does not use a series of MOS devices to modify the gatevoltage of the protection device.

U.S. Pat. No. 3,407,339 of Booher for "Voltage Protection DeviceUtilizing a Field Effect Transistor" uses a series of field effecttransistors with drains coupled to gates for limiting the input voltage.However, Booher does not use this series of transistors to modify thegate voltage of the protection device. This type of protection circuitis not effective for MOS devices avoiding excessive static chargedamage, because the excessive static charges must pass through severaltransistors from the input pad to ground.

SUMMARY OF THE INVENTION

This invention is particularly directed to providing a solution to theproblem of ESD device failure due to the problem of gate oxidebreakdown.

In accordance with this invention, NMOS devices are used as protectiondevices and a series of NMOS with drains coupled to gates modify thegate voltage of a protection device.

In accordance with this invention the breakdown voltage of ESD devicesis reduced to improve ESD performance.

In accordance with this invention, we use NMOS devices as a protectiondevice and a series of NMOS devices with drains coupled to control gatesto modify the gate voltage of the protection device.

Further in accordance with this invention, we use a series of NMOSdevices with drains coupled to control gates to modify the gate voltageof a protected device. The excessive electric charges need pass throughonly one transistor from input pad to ground.

In one aspect of this invention, a MOS electrostatic discharge, ESD,protection circuit for protecting a semiconductor device from ESD damageis formed on a doped silicon semiconductor substrate. A method ofmanufacture of such a device is provided also. The device includes aninput line, an output line, a first node, a second node and a thirdnode, and ground connections. First and second resistors each have aproximal end and a distal end. The first node is connected to the inputline, and the proximal end of the first resistor. The second node isconnected to the proximal end of the second resistor, and the distal endof the first resistor. The third node is connected to the distal end ofthe second resistor, and the output line. The first stage includes afirst MOSFET transistor and a grounded region formed in the substrate.The first MOS transistor has a source/drain circuit connected betweenthe first node and ground, the first MOS transistor having a controlgate electrode connected to ground. The second stage includes a stringof MOSFET transistors connected in a series string with the lasttransistor in the string having a second stage output. The third stageincludes a third stage MOSFET device with a control gate connected tothe second stage output and to the output of the circuit. The source anddrain circuit of the third stage device are connected between the thirdnode and the ground connection. The transistors dissipate power duringan excess energy event to protect devices connected to the output of thecircuit.

Preferably, the first transistor includes a source region and a drainregion on either side of a field oxide region in the substrate, and acontrol gate formed over the field oxide region.

Preferably, the second stage includes a series of FET devices with thesource drain circuits thereof connected in series and the control gateconnected to the input lines closest to the second node.

Preferably, the second stage includes a series of FET devices with thesource drain circuits thereof connected in series and the control gateconnected to the input lines closest to the second node.

In accordance with another aspect of this invention, a MOS electrostaticdischarge, ESD, protection circuit is provided for protecting asemiconductor device from ESD damage. The ESD protection circuit isformed on a doped silicon semiconductor substrate. The circuit includesan input line, an output line, a first node, a second node and a thirdnode, and ground connections, first and second resistors each have aproximal end and a distal end. The first node is connected to the inputline and the proximal end of the first resistor. The second node isconnected to the proximal end of the second resistor and the distal endof the first resistor. The third node connected to the distal end of thesecond resistor and the output line. The first stage includes a firstMOS transistor and a grounded region formed in the substrate. The firstMOS transistor includes a first source region and a first drain regionformed in the substrate, and a control gate electrode therebetween. Thefirst drain region being coupled to the first node. The control gateelectrode and the first source region being connected to the groundedregion. The second stage includes a string of MOSFET transistorsconnected in a series string with the drain and the control gate of thefirst of the string connected to the distal end of the resistor. Thethird stage includes a third stage MOSFET device with a control gateconnected to the output of the second stage and a source and draincircuit thereof connected between the third node and a the groundconnection. The gate electrodes are formed of polysilicon with athickness between 1,000Å and 5,000Å. The source/drain regions have beenion implanted with a dose between 1E15 ions/cm² and 7E15 ions/cm². Thetransistors dissipate power during an excess energy event.

Preferably, the first transistor includes a source region and a drainregion on either side of a field oxide region in the substrate, and acontrol gate formed over the field oxide region.

Preferably, the second stage includes a series of FET devices with thesource drain circuits thereof connected in series and the control gateconnected to the input lines closest to the second node.

Preferably, the second stage includes a series of FET devices with thesource drain circuits thereof connected in series and the control gateconnected to the input lines closest to the second node.

In accordance with still another aspect of this invention, an MOSelectrostatic discharge, ESD, protection circuit is provided forprotecting a semiconductor device from ESD damage, the ESD protectioncircuit formed on a doped silicon semiconductor substrate comprises aseries of stages. A first stage includes a first field effect transistorformed on the substrate, including a first electrode, a secondelectrode, and a control element. An output stage includes an outputfield effect transistor formed on the substrate, including a drainelectrode, a source electrode, and a gate electrode. The secondelectrode and the control element are connected together and grounded tothe substrate. The first electrode, which is connected to a first end ofa first resistor, has its second end connected to a second stageincluding a chain of series connected field effect transistors, having agate and a first electrode connected together, with the first electrodeof the first transistor in the chain connected to the resistor and thesecond electrode of each transistor in the chain connected to the firstelectrode and the control element of the succeeding transistor in thechain, the control element of the final transistor in the chainconnected to the gate electrode of the output stage. A second resistorhas its first end connected to the second end of the first resistor, thesecond resistor has a second end. The drain electrode of the third stageconnected to the second end of the second resistor and the sourceelectrode of the third stage grounded to the substrate.

Preferably, the first transistor includes a drain region connected tothe first electrode and a source region connected to the secondelectrode, the drain region and the source region being on either sideof a field oxide region in the substrate, and the control element formedover the field oxide region.

Preferably, the second stage includes a series of FET devices havingsource drain circuits connected in series and the control gate connectedto the input lines closest to the second node.

Preferably, the second stage includes a series of FET devices with thesource drain circuits connected in series and the control gate connectedto the input lines closest to the second node.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects and advantages of this invention areexplained and described below with reference to the accompanyingdrawings, in which:

FIG. 1 shows a simple two stage ESD protection circuit which isrelatively likely to fail due to gate oxide breakdown, when voltagesexceed a critical limit.

FIG. 2 shows a preferred ESD protection circuit in accordance with thisinvention with a three stage ESD circuit.

FIG. 3 shows a combined schematic circuit diagram and a cross section ofthe device of FIG. 2.

FIGS. 4 and 5 show a second preferred ESD protection circuit inaccordance with this invention including three stages as in FIGS. 2 and3. The first and third stages of the circuit are identical to the firstand third stages of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a simple two stage ESD protection circuit withlimited capacity to protect against large voltages. A contact pad P1connected via line L1 through node N1 to the proximal end (close end) ofresistor R1. Node N1 is connected via line L2 to the drain of an NMOSMOSFET device T1, the source of which is connected to ground throughline L4. The control gate CG1 of device T1 is connected via line L3 toground or which remains floating. The distal end of resistor R1 isconnected via line L5 through node N2 as indicated by the arrow to theinput buffer circuits (not shown) to be protected. Connected betweennode N2 and ground is a grounded gate NMOS device T2. The drain ofdevice T2 is connected to node N2 and the source of device T2 isconnected through line L7 to ground. The control gate CG2 of device T2is also connected to ground through line L7.

The device T2 operates when line L7 is connected to ground. When thevoltage of node N2 is higher than junction breakdown voltage of deviceT2, the electric charges pass through device T2.

A hypothetical alternative to FIG. 1 would be to connect the controlgate CG2 of device T2 to the drain D rather than to source S. In thecase of connecting the control gate CG2 to line L6 and to the drain ofdevice T2, device T2 would always be turned on as the voltage of thedrain would always be higher than the threshold voltage of device T2.Accordingly, the device T2 would be very leaky under normal operationvoltage and would not function adequately.

FIRST PREFERRED EMBODIMENT OF INVENTION

FIG. 2 shows a preferred ESD protection circuit in accordance with thisinvention with a contact pad P11 connected to the first stage of a threestage ESD circuit.

FIRST STAGE

The first stage is connected to contact pad P11 via line L11 through anode N11 to the proximal end of resistor R11. Node N11 is connected vialine L12 to the drain of an NMOS MOSFET device T11, the source of whichis connected to ground through line L14. The control gate CG11 of deviceT11 is connected via line L13 to line L9 which in turn is connected toground.

The distal end (far end) of resistor R11 is connected via line L15 tonode N21a, which is connected to line L14 and to the proximal end ofresistor R21 which is connected at its distal end to line L18 which, asindicated by the arrow is connected to the input buffer of the circuitto be protected (not shown for convenience of illustration.)

SECOND STAGE

The drain of device T21a is connected through line L16a to node N21a'.Node N21a' is connected via line L16 to node N21a. Node N21a' is alsoconnected via line L17a to the control gate CG21a of device T21a. Thesource region S21a of device T21a is connected through line L16b to nodeN21b. Node N21b is connected to the drain region D21b and via line L17bto control gate CG21b. Thus the drain D21b and the control gate CG21b ofdevice T21b are connected to the source of device T21a, as well.

The source S21b of device T21b is connected through line L16c to thedrain and the control gate of an NMOS FET device (T21c not shown.) Thesequence (string) of FET devices can continue as an array of FET devicesT21d . . . T21n-1 which also are not shown, depending upon the number ofstages of devices required.

At the end of the string of devices, there is a line L19 which isconnected from the source S of transistor T21n, (which is the last FETin the chain T21a, T21b, . . . T21n) to the control gate CG31 of deviceT31.

The manner in which a circuit in accordance with this invention workswith the drains connected to the control gates in FIG. 2 is describednext. The voltage of the source of FET device T21n will be the voltageof node N21a minus the sum of the threshold voltages of the interveningFET devices T21a, T21b . . . and T21n, assuming that the voltage at nodeN21a is larger than the sum of threshold voltage of devices T21a, T21b .. . and T21n. In addition, the voltage of the source of device T21n willbe nearly zero normally, if the voltage of node N21a is less than thesum of the threshold voltage of devices T21a, T21b . . . and T21n. Bychoosing the proper quantity of devices T21a, T21b . . . and T21n, thesum of the threshold voltages T21 . . . T21n and the protection deviceT31 is just larger than normal operation voltage. As a result, theprotection device will not be turned on under normal operationconditions, yet this protection device will be turned on if the voltageon node N21a is higher than normal operation voltage. The junctionbreakdown voltage of the protection device T31 will be reduced if thisprotection device is turned on. The efficiency of the protection devicecan be enhanced by reduction of its junction breakdown voltage.

The manner of operation of the series string of transistors T21a-T21n isdescribed above.

THIRD STAGE

The third stage of the circuit in FIG. 2 is connected from the source oftransistor T21n via line L19 which connects to the control gate CG31 ofFET T31.

In the third stage, FET T31 has its drain D31 connected by line L20 vianode N31 to line L18. The source S31 of FET T31 is connected by line L21to ground.

Thus, between node N31 and line L21, connected to ground, are the drainand the source of NMOS device T31.

FIG. 3 shows a combined schematic circuit diagram and a cross section ofthe device of FIG. 2.

The device 7 includes a P-doped silicon semiconductor substrate 8 (orP-well) upon the surface of which a plurality of field oxide (FOX)regions 10a, 10b, 10c, 10d, 10e, 10f . . . 10n-1, 10n have been formedfrom left to right.

FIRST STAGE

Formed between FOX regions 10a and 10c is transistor T11 which includesthe N+ drain region D11 between FOX regions 10a and 10b, and the N+source region S11 between FOX regions 10b and 10c. Over the FOX region10b is formed the control gate CG 11 between the N+ drain region D11 andthe N+ source region S11. Beneath the FOX region 10b is the channel C11of transistor T11. Drain region D11 is connected to line L12. Sourceregion S11 is connected to line L14, which is connected to line L14which connects through node N12 and node N12' to P+ region G1 betweenFOX regions 10c and 10d. Region G1 is P+ in a P-substrate or P-well 8 sothe region G1 is grounded to the potential of the substrate 8. The P+region G1 in this circuit serves as a ground contact region. Controlgate CG 11 is connected via node N12 and line L14 to both source regionS11 and P+ region G1.

SECOND STAGE

Formed between FOX regions 10d and 10e is transistor T21a which includesthe N+ drain region D21a, the N+ source region S21a, the channel C21aand the control gate CG21a. Between the drain region D21a and the sourceregion S21a is the channel C21a over which is formed the control gateCG21a. Drain region D21a is connected to line L16a, which is connectedvia line L17a to control gate CG21a. Source region S21a is connected toline L16b.

Formed between FOX regions 10e and 10f is transistor T21b which includesthe N+ drain region D21b, the N+ source region S21b, the channel C21band the control gate CG21b. Between the drain region D21b and the sourceregion S21b is the channel C21b over which is formed the control gateCG21b. Drain region D21b is connected via node N21b to line L16b, whichis connected to source S21a of the previous transistor in the chain.Source region S21b is connected to line L16c.

Formed to the right of FOX region 10f would be transistor T21c which isnot shown since the drawing shows but a fragment of the overall devicewith an intermediate section deleted for convenience of illustration.

Formed farther to the right of FOX region 10f is a portion of transistorT21n which includes the source region S21n, (located to the left of FOXregion 10n-1) connected through line L16n+1 to line L19 to the controlgate CG31 of the third stage of the circuit.

THIRD STAGE

Formed between FOX regions 10n-1 and 10n is transistor T31 whichincludes the N+ drain region D31 to the right of FOX region 10n-1 andthe N+ source region S31 to the left of FOX region 10n, farthest to theright. Between the N+ drain region D31 and the N+ source region S31 isformed the control gate CG31 over the channel region C31. Drain regionD31 is connected via line L20 to node N31, which connects to line L18 tothe output of the device of FIG. 3, as described above in connectionwith FIG. 2. Source region S31 is connected via line L14 to ground.

The control gate CG31 is connected via line L19 to node N21n and sourceregion Sn as described above to lower node N31 to ground when anovervoltage has appeared at node N21a, which has caused the combinedthreshold voltages of the devices T21a, T21b . . . T21n to be exceeded.

Operation of the circuit of FIGS. 2 and 3.

a. Negative ESD charge: Referring to FIG. 3, there is a forward bias ofthe N+ regions (Source/Drain regions) with respect to substrate (orP-well) 8, so that charge is easily conducted to ground due to theforward bias.

The charge flows from n+ drain region D11 in FIG. 3 and where the chargeflows to P+ region G1 in FIG. 3.

b. Positive ESD charge: Positive charge passes through the protectiondevice to ground by device breakdown. In this sense junction devicebreakdown occurs when a sufficiently high voltage is reached at whichthe device conducts charge in the reverse direction with a very largecurrent. To prevent excessive heating of the device during junctionbreakdown, maximum power must be limited to minimize heating to a levelat which no damage occurs. In the device of this invention, thebreakdown voltage of the protection device is reduced by modifying thegate voltage.

In accordance with this invention, to reduce the breakdown voltage of anESD protection device, no ground gate ESD protection device is used; andin the second stage of a three stage device NMOS devices are arranged ina series string which is connected between an I/O pin and the gate of anESD device in the third stage of the circuit.

The breakdown voltage V_(BD) with a circuit in accordance with thisinvention is as follows:

    V.sub.BD >n * V.sub.t ≧V.sub.D

n: number of stages of devices in ESD chain

V_(BD) : breakdown voltage of gate oxide

V_(D) : normal operation voltage of circuit. (where V_(DD) is drainsupply voltage)

V_(t) : Threshold voltage

where the minimum value of n=2.

For example, if n=2, then

    V.sub.BD >2 * V.sub.t ≧V.sub.D

For example, if n=3, then

    V.sub.BD >3 * V.sub.t ≧V.sub.D

* Device breakdown voltage is a function of gate voltage.

The device breakdown voltage of an NMOS * with positive gate voltage islower than the ground gate transistor's breakdown voltage.

Process of forming device of FIG. 3.

The semiconductor fabrication process (recipe) for forming the devicesof FIG. 3 is a conventional CMOS process. Stages 1, 2 and 3 are all madeby the same process which is as follows:

Fox Mask for Ion Implanting

Dosage Levels 1E12 ions/cm² TO 6E13 ions/cm²

Power Levels 20 keV to 200 keV

GOX (Gate oxide:silicon dioxide),

Method of Forming:

Thermal growth or CVD silicon dioxide

Thickness Range 40Å to 200Å

Gate Electrodes

Polysilicon Blanket

Thickness Range 1,000Å to 5,000 Å.

Mask Pattern

Etch Polysllicon with CHF₃ /CF₄ /HBr/C1₂ /Ar/He/O₂

Remove Mask

S/D Ion Implant Steps

Dosage Levels 1E15 ions/cm² to 7E15 ions/cm²

Power Levels 10 keV to 100 keV

Stage 1 Ground (G1) ion implant steps

Dosage Levels 1E15 ions/cm² to 7E15 ions/cm²

Power Levels 10 keV to 100 keV

SECOND PREFERRED EMBODIMENT OF INVENTION

FIGS. 4 and 5 show a second preferred ESD protection circuit inaccordance with this invention including three stages as in FIGS. 2 and3. The first and third stages of the circuit are identical to the firstand third stages of FIGS. 2 and 3, so the description will be notinclude a description of the first stage. A brief description of theThird Stage of the circuit follows the description of the modifiedSecond stage.

SECOND STAGE

It should be noted that the second stage has FET devices which sharesource/drain regions between successive devices. The string of FETsT41a, T41b, T41c, . . . T41n comprises a set of combined source/drainregions R41b, R41c, R41d, . . . R41n-1. For example, the N+ region R41bserves at one end thereof as the source region of transistor T41a and atthe opposite end thereof as the drain region of transistor T41b as shownin FIG. 5, thereby eliminating the need for a wired connectedtherebetween.

Referring to FIGS. 4 and 5, line L15 is connected from node N21a throughwhich it passes to line L16. Node N21a' connects from line L16 to linesL46a and L47a. Line 46a connects node N21a' to the drain of N+ dopedregion R41a of device T41a. Line L47a connects to the control gate CG41aof NMOS FET device T41a, having as its source a portion of N+ regionR41b in substrate 8.

The drain portion of N+ doped region R41b of device T41b is connectedthrough line L47b to the control gate CG41b of device T41b, having asits source a portion of N+ region R41c in substrate 8.

The drain portion of N+ doped region R41c of device T41c is connectedthrough line L47c to the control gate of an NMOS FET device T41c, havingas its source a portion of N+ region R41d in substrate 8.

The drain region of N+ doped R41d of device T41d is connected throughline L47d to the control gate of an NMOS FET device (device T41d notshown.)

The sequence (string) of FET devices can continue as an array of FETdevices T41e . . . T41n-1 which also are not shown, depending upon thenumber of stages of devices required.

At the end of the string of devices, there is a line L47n from thesource portion of N+ region (R41n-1 not shown) of a device (FET T41n-1,not shown for convenience of illustration) connected to the control gateCG41n of device T41n, which is the last device in the second stage.

From the source in doped N+ region R41n, there is a line L47n+1 ofdevice T41n which connects to the input line L19 of the third stage.

THIRD STAGE

The drain of device R41n (comprising N+ doped region R41n) is connectedby line L47n+1 which becomes line L19 to control gate CG31 of deviceT31, which has the same structure as the third stage of FIGS. 2 and 3.Referring to FIG. 5, on the right side of the region R41n is located aFOX region 10n-1, next to which is formed the N+ doped drain region D31on the left of control gate CG31. Drain region D31 is connected by lineL20 to node N31, which connects to line L18. To the right side ofcontrol gate CG31 is the N+ source region S31 (connected via line L21 toground), adjacent to FOX region 10n.

Referring again to FIG. 5 a combined schematic circuit diagram and across section of the device of FIG. 4 are shown.

In FIG. 5, the string of devices is formed between the two FOX regions10d and 10n-1 without any other FOX regions in between. Region R41a ofan NMOS FET device T41a is formed to the right of FOX region 10d, withthe channel C41a of device T41a next followed by the N+ doped regionR41b thereof.

Above the channel C41a is the control gate CG41a of device T41a. LineL15 is connected through node N21a where it joins lines L47a whichconnect to the N+ region D21a of device T41a and the control gate CG41a.

To the right of the N+ doped region R41b is the channel C41b of NMOS FETdevice T41b. Above channel C41b is found control gate CG41b of deviceT41b. N+ doped region R41b is connected through line L47b to the controlgate CG41b of device T41b.

To the right of channel C41b is the N+ doped region R41c of device T41cwhich is connected through line L47c to the control gate CG41c of anNMOS FET device T41c.

The N+ doped region R41d of device T41d (not shown) is connected throughline L47d to the drain and the control gate of an NMOS FET device T41d(not shown.)

The sequence (string) of FET devices can continue as an array of FETdevices T41e . . . T41n-1 which also are not shown, depending upon thenumber of stages of devices required.

Near the end of the string of devices, there is a line L47n from thesource S41n-1 (not shown) of device FET T41n-1 (also not shown)connected to the control gate CG41n of device T41n.

Process of forming device of FIGS. 4 and 5

The semiconductor fabrication process (recipe) for forming the devicesof FIGS. 4 and 5 is a conventional CMOS process which is the same as theprocess for FIG. 3. Stages 1, 2 and 3 are all made by the same process.

Fox Mask for Ion Implanting

Dosage Levels 1E12 ions/cm² TO 6E13 ions/cm²

Power Levels 20 keV to 200 keV

GOX (Gate oxide:silicon dioxide),

Method of Forming:

Thermal growth or CVD silicon dioxide

Thickness Range 40Å to 200Å

Gate Electrodes

Polysilicon Blanket

Thickness Range 1,000Å to 5,000 Å.

Mask Pattern

Etch Polysilicon with CHF₃ /CF₄ /HBr/Cl₂ /Ar/He/O₂

Remove Mask

S/D Ion Implant Steps

Dosage Levels 1E15 ions/cm² to 7E15 ions/cm²

Power Levels 10 keV to 100 keV

Stage 1 Ground (G1) ion implant steps

Dosage Levels 1E15 ions/cm² to 7E15 ions/cm²

Power Levels 10 keV to 100 keV

Summary

The transistors conduct at high levels of current in order to protectagainst overvoltage and to dissipate power during an excess energy eventto protect devices connected to the output of the circuit.

While this invention has been described in terms of the above specificembodiment(s), those skilled in the art will recognize that theinvention can be practiced with modifications within the spirit andscope of the appended claims, i.e. that changes can be made in form anddetail, without departing from the spirit and scope of the invention.Accordingly all such changes come within the purview of the presentinvention and the invention encompasses the subject matter of the claimswhich follow.

Having thus described the invention, what is claimed as new anddesirable to be secured by Letters Patent is as follows:
 1. An MOSelectrostatic discharge, ESD, protection circuit for protecting asemiconductor device from ESD damage, said ESD protection circuit formedon a doped silicon semiconductor substrate comprising:an input line, anoutput line, a first node, a second node and a third node, and groundconnections, first and second resistors each having a proximal end and adistal end, said first node connected to said input line, and saidproximal end of said first resistor, said second node connected to saidproximal end of said second resistor, and said distal end of said firstresistor, said third node connected to said distal end of said secondresistor, and said output line,said first stage including a first MOSFETtransistor and a grounded region formed in said substrate, said firstMOS transistor having a source/drain circuit connected between saidfirst node and ground, said first MOS transistor having a control gateelectrode connected to ground, said second stage including a string ofMOSFET transistors connected in a series string with the last transistorin said string having a second stage output, said third stage includinga third stage MOSFET device with a control gate connected to said secondstage output and a source and drain circuit connected between said thirdnode and a said ground connection,whereby said transistors dissipatepower during an excess energy event.
 2. The protection device of claim 1wherein said first transistor includes a source region and a drainregion on either side of a field oxide region in said substrate, and acontrol gate formed over said field oxide region.
 3. The protectiondevice of claim 2 wherein said second stage includes a series of FETdevices with the source drain circuits thereof connected in series andthe control gate connected to the input lines closest to said secondnode.
 4. The protection device of claim 1 wherein said second stageincludes a series of FET devices with the source drain circuits thereofconnected in series and the control gate connected to the input linesclosest to said second node.
 5. An MOS electrostatic discharge, ESD,protection circuit for protecting a semiconductor device from ESDdamage, said ESD protection circuit formed on a doped siliconsemiconductor substrate comprising:said device including a silicondioxide gate oxide layer formed on said semiconductor substrate with athickness between 40Å and 200 Å, an input line, an output line, a firstnode, a second node and a third node, and ground connections, first andsecond resistors each having a proximal end and a distal end, said firstnode connected to:said input line, and said proximal end of said firstresistor, said second node connected to:said proximal end of said secondresistor, andsaid distal end of said first resistor, said third nodeconnected to:said distal end of said second resistor, and said outputline, said first stage including a first MOS transistor and a groundedregion formed in said substrate, said first MOS transistor including afirst source region and a first drain region formed in said substrate,and a control gate electrode therebetween, said first drain region beingcoupled to said first node, said control gate electrode and said firstsource region being connected to said grounded region, said second stageincluding a string of MOSFET transistors connected in a series stringwith the drain and the control gate of the first of said stringconnected to the distal end of said first resistor, said third stageincluding a third stage MOSFET device with a control gate connected tothe output of said second stage and a source and drain circuit thereofconnected between said third node and a said ground connection, saidgate electrodes being formed of polysilicon with a thickness between1,000Å and 5,000 Å, said source/drain regions having been ion implantedwith a dose between 1E15 ions/cm² and 7E15 ions/cm², whereby saidtransistors dissipate power during an excess energy event.
 6. Theprotection device of claim 5 wherein said first transistor includes asource region and a drain region on either side of a field oxide regionin said substrate, and a control gate formed over said field oxideregion.
 7. The protection device of claim 6 wherein said second stageincludes a series of FET devices with the source drain circuits thereofconnected in series and the control gate connected to the input linesclosest to said second node.
 8. The protection device of claim 5 whereinsaid second stage includes a series of FET devices with the source draincircuits thereof connected in series and the control gate connected tothe input lines closest to said second node.
 9. An MOS electrostaticdischarge, ESD, protection circuit for protecting a semiconductor devicefrom ESD damage, said ESD protection circuit formed on a doped siliconsemiconductor substrate comprising:a first stage including a first fieldeffect transistor formed on said substrate, including a first electrode,a second electrode, and a control element, an output stage including anoutput field effect transistor formed on said substrate, including adrain electrode, a source electrode, and a gate electrode, said secondelectrode and said control element being connected together and groundedto said substrate, said first electrode connected to a first end of afirst resistor having its second end connected to a second stageincluding a chain of a plurality of series connected field effecttransistors, having a gate and a first electrode connected together,with the first electrode of the first transistor in said chain connectedto said resistor and the second electrode of each transistor in saidchain connected to the first electrode and the control element of thesucceeding transistor in said chain, the control element of the finaltransistor in said chain connected to said gate electrode of said outputstage, a second resistor having its first end connected to said secondend of said first resistor, said second resistor having a second end,said drain electrode of said third stage connected to said second end ofsaid second resistor and said source electrode of said third stagegrounded to said substrate.
 10. The device of claim 9 wherein said firsttransistor includes a drain region connected to said first electrode anda source region connected to said second electrode, said drain regionand said source region being on either side of a field oxide region insaid substrate, and said control element formed over said field oxideregion.
 11. The protection device of claim 10 wherein said second stageincludes a series of FET devices having source drain circuits connectedin series and the control gate connected to the input lines closest tosaid second node,said device including a silicon dioxide gate oxidelayer formed on said semiconductor substrate with a thickness between40Å and 200 Å, said gate electrodes being formed of polysilicon with athickness between 1,000Å and 5,000Å, and said source/drain regionshaving been ion implanted with a dose between 1E15 ions/cm² and 7E15ions/cm².
 12. The protection device of claim 9 wherein said second stageincludes a series of FET devices with the source drain circuitsconnected in series and the control gate connected to the input linesclosest to said second node.